Following Turkey's purchase of Russian S-400 defence systems earlier this month, the United States said on Wednesday that it would remove Turkey from the F-35 fighter programme.
Despite repeated warnings from the United States, Turkey began receiving the initial parts of the defence system on June 12.
Why does the US oppose the deal?
The US claims that the S-400 system is a threat to their F-35 fighter jet system and is incompatible with NATO's systems.
"The S-400 is a computer. The F-35 is a computer. You don't hook your computer to your adversary's computer and that's basically what we would be doing," said Katie Wheelburger, American acting assistant secretary of defence for international security affairs, earlier this year.
Another reason that the US opposes this deal so fervently is that it contravenes their "Countering America's Adversaries through Sanctions Act" (CAATSA). This legislation requires the US state to impose sanctions on any country that purchases military equipment from Russia.
Why did Turkey buy the system?
Turkey claims that the S-400 system is a strategic defence requirement against threats posed in border countries Syria and Iraq.
Ankara has also said that it hopes to work with Russia in developing next-generation S-500 missile systems — co-production and technology transfer in this deal will be of benefit to them in the long run.
Turkey also said that the Russian system met its expectations in terms of price and technology, whereas the United States and Europe had not presented a viable alternative.
The system does not pose a direct threat to the US military, Turkey claimed. Ankara has proposed setting up a group to investigate the United States' concerns about the potential impact of the system on the F-35 fighter jets — this group could include NATO, it said.
While defence companies in Turkey may face losses after their removal from the F-35 programme, the industry will emerge stronger as a result, said Ismail Demir, president of defence industries in Turkey.
The defence chief also said on Twitter that the removal of Turkey from the fighter jet programme will result in costs of $7 million to $8 million (€6,240,500 to €7,132,000) for the remaining countries in the programme.
Demir also said that Turkey would not purchase foreign defence equipment in the future unless it was absolutely necessary but did not elaborate on this further.
Relations between the allies are tense. Turkey is suffering because of sanctions the US has imposed on Iran, its leading supplier of gas and oil. Ankara is also unhappy with US support of the Kurdish YPG militia in Syria, which Turkey views as a terrorist organisation.
With relations already at a breaking point, some see the S-400 deal as evidence that Ankara is moving further under the sphere of Russian influence.